There is an increasing demand for high quality premium coke for the manufacture of large graphite electrodes for use in electric arc furnaces employed in the steel industry. The quality of premium coke used in graphite electrodes is often measured by its coefficient of thermal expansion which may vary from as low as -5 to as high as +8 centimeters per centimeter per degree centigrade times 10.sup.-7. Users of premium coke continuously seek graphite materials having lower CTE values. Even a small change in CTE can have a substantial effect on large electrode properties.
Premium coke is manufactured by delayed coking in which heavy hydrocarbon feedstocks are converted to coke and lighter hydrocarbon products. In the process the heavy hydrocarbon feedstock is heated rapidly to cracking temperatures and is fed into a coke drum. The heated feed soaks in the drum in its contained heat which is sufficient to convert it to coke and cracked vapors. The cracked vapors are taken overhead and fractionated with the fractionator bottoms being recycled to the feed if desired. The coke accumulates in the drum until the drum is filled with coke at which time the heated feed is diverted to another coke drum while the coke is removed from the filled drum. After removal the coke is calcined at elevated temperatures to remove volatile materials and to increase the carbon to hydrogen ratio of the coke.
In the manufacture of large graphite electrodes, calcined premium coke particles obtained from the delayed coking process are mixed with pitch and then baked at elevated temperatures to carbonize the pitch.
The delayed coking operation is a semi-continuous process in which the feed material is introduced continuously to the coke drum during the coking cycle. If the coking cycle lasts for say 30 hours, the feed material first introduced to the coke drum is subjected to coking conditions for this period of time. Each succeeding increment of feed, however, is coked for a lesser period of time, and the final portion of feed material introduced to the coke drum is subjected to coking conditions only for a relatively short period of time. In view of this, it is understandable that problems are encountered in obtaining coke product which is homogeneous. Coke produced near the top of the drum, where reaction times are short, generally has different physical properties, e.g., CTE and Crush Index, than coke produced in the remainder of the drum. Coke which is not uniform presents a problem for graphite producers in a number of ways. Pitch demand, coke sizing, and ultimate electrode performance all become difficult to predict if coke properties are not consistent.